1. Field of the Invention
The present invention relates to methods for manufacturing semiconductor devices, and more particularly to a technology effectively applicable to a method for manufacturing a semiconductor device having a trench gate structure.
2. Description of the Related Art
As switching devices used in inverters or converters in power conversion devices, insulated-gate bipolar transistors (IGBT) having a trench gate structure are known. Such an IGBT having a trench gate structure, if a capacitance between a gate and a collector is large, requires a considerable amount of charge/discharge time and may cause an increase in generated loss when shifting from an ON state to an OFF state. The generated loss is a sum of steady-state loss determined depending on an ON-state voltage and switching loss upon the ON state and the OFF state. It is therefore essential to reduce the capacitance between the gate and the collector causing such switching loss.
JP 2007-074006A discloses a dummy trench structure for reducing a capacitance between a gate and a collector. In the dummy trench structure, an electrode provided in a predetermined trench (dummy trench) of a plurality of trenches is electrically connected to an emitter electrode. The structure disclosed in JP 2007-074006A can decrease an absolute value of a gate capacitance, and also decrease a capacitance between the gate and the collector in the IGBT particularly including a floating layer.
However, the dummy trench structure disclosed in JP 2007-074006A, in which the electrode provided in the dummy trench is electrically connected to the emitter electrode, cannot easily screen out defective portions such as abnormal shape of the dummy trench or quality-degradation of an insulating film inserted between the dummy trench and the electrode by applying voltages.